ELECTRON-TEMPERATURE MEASUREMENTS FROM OPTICALLY GRAY 3RD-HARMONIC ELECTRON-CYCLOTRON EMISSION IN THE DIII-D TOKAMAK

In many high-performance discharges in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] high electron density prevents determining the electron temperature from the second harmoni c electron cyclotron emission (ECE). A technique for obtaining central electron temperatures from optically gray third harmonic ECE is prese nted that does not require knowledge of the reflective properties of t he vessel wall. The temperature values derived from ECE spectra measur ed with an absolutely calibrated Michelson interferometer agree with i ndependent measurements by Thomson scattering. As part of this work, a method of determining the optical depth of third harmonic frequencies in a low aspect ratio tokamak is also demonstrated. The optical depth measurements are in agreement with calculations correct to first orde r in the Larmor radius. (C) 1996 American Institute of Physics.